1. Field of the Invention
The present invention relates to a sensor for measuring a dynamic quantity by utilizing capacitance change, and more particularly to a semiconductor pressure sensor.
2. Description of the Related Art
Conventional techniques relevant to a pressure sensitive capacitance element of this invention are described in JP-A-8-501156 which presents an electrostatic capacitance type pressure gauge manufactured through etching of a sacrificial layer, such as shown in FIG. 20. This pressure gauge is constituted of a pressure sensitive capacitance element 2 whose capacitance value changes with an applied pressure, and a reference capacitance element 3 although whose capacitance value is similar to that of the capacitance element 2 it does not change with the applied pressure. The structure and operation principle of this pressure gauge will be described.
Referring to FIG. 20, a fixed electrode 6 is formed on the surface of a silicon substrate 1, and a movable electrode 5 made of a polysilicon film is formed over the fixed electrode 6, with a space 7 being interposed therebetween. This space 7 was formed by etching and removing a sacrificial film formed in the space 7, through etch channels 12 partially formed through the variable electrode 5. In order to vacuum seal this space 7, a sealing film 9 made of a silicon oxide film is formed closing the etch channels 12. The space 7 in this state forms a pressure reference chamber which was vacuum sealed. This space 7, fixed electrode 6 formed on the substrate in the pressure reference chamber, and movable electrode 5 made of the polysilicon film, constitute a capacitor. The reference capacitance element 3 has a similar structure to that of the pressure sensitive capacitance element 2. However, the sealing film 9 on the variable electrode 5 of the reference capacitance element 3 is not removed to increase a diaphragm rigidity.
As an external pressure changes, the variable electrode 5 of the pressure sensitive capacitance element 2 displaces by an amount corresponding to a difference between the external pressure and the pressure in the pressure reference chamber. Therefore, the gap between the variable electrode 5 and fixed electrode 6 changes and the capacitance changes. Since the reference capacitance element 3 has a high film rigidity, the variable electrode 5 will not be displaced by a pressure change and the capacitance thereof will not change.
The capacitance value of the pressure sensitive capacitance element 2 will be described quantitatively. An initial capacitance value Cs0 of the pressure sensitive capacitance element 2 is given by:
Cs0=∈Ss/d0xe2x80x83xe2x80x83(1)
where ∈ is a dielectric constant of a material in the electrode gap, Ss is an electrode area, and d0 is an initial electrode gap. If a pressure P displaces the electrode gap of the pressure sensitive capacitance element 2 by xcex94d from the initial value d0 and if xcex94d=kP where k is a diaphragm spring constant, then:
Cs=∈Ss/(d0xe2x88x92xcex94d)=∈Ss/(d0xe2x88x92kP)xe2x80x83xe2x80x83(2)
A capacitance value Cr of the reference capacitance element 3 is given by:
Cr=Cr0=∈Sr/d0xe2x80x83xe2x80x83(3)
where ∈ is a dielectric constant of a material in the electrode gap of the reference capacitance element 3, Sr is an electrode area, and d0 is an initial electrode gap.
In a general differential capacitance type pressure sensor, a difference xcex94C between these two elements is converted into a voltage xcex94V by a C-V converter circuit and amplified by and output from an amplifier.
A first known example of the C-V converter circuit is described in JP-A-5-231973, and the circuit diagram thereof is shown in FIG. 21. This circuit is structured based upon a generally known switched capacitor circuit. An output of this circuit is given by:
Vout={(Csxe2x88x92Cr)/Ci}xc2x7Vccxe2x80x83xe2x80x83(4)
The output changes with a capacitance difference of the pressure sensitive capacitance element 2 and reference capacitance element 3. As apparent from the equations (2) and (4), the output voltage Vout is nonlinear relative to the pressure P. This circuit is therefore associated with a problem that the output is required to be corrected in order to obtain a linear output.
A second known example of a C-V converter circuit which can solve this problem is described in Sensors and Actuators A60 (1970) pp. 32-36. In this circuit, an integration capacitance of an operational amplifier is used as the pressure sensitive capacitance element whose electrostatic capacitance changes with a pressure, and the charge quantity accumulated in this element is converted into a voltage signal. The circuit structure is shown in FIG. 22, and an output is given by the following equation (5):
Vout=xe2x88x92(Cr/Cs)xc2x7VBxe2x80x83xe2x80x83(5)
By using the equations (1), (2) and (3), the equation (5) becomes:
Vout=(Cr0/Cs0)xc2x7{(kP/d0)xe2x88x921}xc2x7VBxe2x80x83xe2x80x83(6)
Since the output voltage changes proportionally with the pressure P, a pressure detection precision can be improved.
Among various kinds of applications of a pressure sensor, a pressure sensor used for controlling a vehicle engine is required to have a high pressure detection precision and high noise resistance. A high precision capacitance type C-V converter circuit like the second known example is therefore preferable to be adopted. In order to improve the noise resistance, it is effective to have a large output signal xcex94V of a pressure gauge and lower the amplification factor of the amplifier, from the viewpoint of an S/N ratio.
With the above-described capacitance type C-V converter circuit, a C-V conversion efficiency is determined by an initial capacitance value ratio between those of the pressure sensitive capacitance element and reference capacitance element, as indicated by the equation (6). Conventionally, this ratio has been set nearly to 1 and has not been used as a sensitivity adjustment parameter. Therefore, in order to have a large output of the pressure gauge, it is necessary to increase a displacement amount by lowering a diaphragm rigidity, or to increase the capacitance change xcex94C itself by narrowing the electrode gap. However, lowering the diaphragm rigidity too much or narrowing the electrode gap may result in a contact between the diaphragm and substrate in the range of the measuring pressure. The degree of design freedom is therefore limited.
The present invention has been made to solve the above-described problems. It is an object of the invention to provide a pressure gauge of the type that a pressure is detected by using a capacitance ratio between those of a pressure sensitive capacitance element and reference capacitance element, the pressure gauge being capable of increasing a C-V conversion efficiency and improving a sensor output S/N ratio by using a simple method.
In order to achieve the above object, according to the main feature of the invention, a semiconductor pressure sensor comprises: a pressure sensitive capacitance element having an electrostatic capacitance Cs changing with a pressure to be detected; a reference capacitance element having an electrostatic capacitance Cr not changing with the pressure; and means for detecting the pressure by outputting a signal corresponding to a ratio between the capacitances Cs and Cr, wherein an initial value Cr0 of the capacitance Cr and an initial value Cs0 of the capacitance Cs are defined by 1.2 less than Cr0/Cs0 less than 1.8.